The reactor core for commercial nuclear reactors of the type used for generating electric power, contain about 200 fuel assemblies disposed in a predetermined pattern in the reactor core. Each fuel assembly comprises a multiplicity of fuel rods interspersed with a small number of control rods guide thimbles which are hollow tubes designed to receive control rods adjustably positioned therein to vary the reactor core heat energy output during reactor operations. Although all assemblies do not need control rods, each assembly nevertheless is equipped with control rod guide thimbles to accommodate control rods in the event the assembly later is placed in a control position in the reactor core. Since reactor coolant can flow upwardly through the hollow control rod guide thimbles in fuel assemblies which do not contain control rods, a plug conventionally is positioned in the top of each unused guide thimble. These plugs are connected to a common plate removably positioned on the top of each assembly thus permitting removal of all plugs from one fuel assembly in one operation. In carrying out their function precluding passage of coolant through the unused guide thimbles, the plugs are forcefully held in a static position and accordingly cause the coolant to flow into contact with the heat generating fuel rods.
After a predetermined period of reactor operation, the fuel assemblies are rearranged in the reactor core. Spent fuel assemblies are removed and replaced by new assemblies, and partially spent fuel assemblies are moved to different positions in the core. During the course of carrying out this refueling operation, plugs in the guide tubes must be removed and reinserted in different assemblies. According to one prevailing practice, the plugs are removed as a unit from each assembly after its removal from the reactor and during the time it is in the assembly transfer system, or in a change fixture. In either event, a long handled tool especially designed for plug removal purposes is utilized to effect plug removal from a particular assembly. This operation is exceedingly time consuming because the assembly must remain submerged in a neutron absorbing liquid or coolant during the refueling operation and the operators must therefore work under poor visibility conditions. Since the tool is approximately 30 feet long, its unwieldiness causes tool-thimble plug alignment problems, during the time of plug removal and insertion. Further, the plug removal hoist mechanism must frequently move between the reactor and fuel assembly transfer area as plugs are removed from some assemblies and replaced in other assemblies located in the reactor.
The total time applied to plug removal and replacement in control rod guide thimbles accounts for an important part of the complete refueling operation. Since an electric power generating utility cannot generate and sell electric power during reactor down time for refueling, it is apparent the need exist for improved apparatus for removing and replacing guide thimble plugs in substantially less time than that presently required and in a more reliable, safe and efficient manner.